1. Field of the Invention
The present invention relates to a process for the recovery of uranium contained in an impure phosphoric acid. More particularly, the invention concerns the concentration and purification of uranium extracted from a wet process phosphoric acid, i.e. from phosphoric acid produced by the acidulation of phosphate rock.
2. Discussion of the Prior Art
Phosphate rock naturally contains small amounts of uranium (from about 50 to 400 p.p.m.). The uranium is dissolved during acidulation of the phosphate rock and remains in the phosphoric acid solution thus produced. Although the concentration of uranium in such solutions is low, the wet-process phosphoric acid is a valuable source of uranium because of the vast quantities of phosphate rock mined each year and processed to recover high-phosphate-containing fertilizer.
A number of attempts have been made in the past to develop commercially feasible methods of recovering uranium from such aqueous solutions. Thus, it is known to recover uranium from aqueous solutions in which it is present in low concentrations, by separating the other potentially usable components of the minerals treated by means of a liquid-liquid extraction assembly and by chemical treatment for the purpose of isolating the uranium and recovering it in the form of a high purity U.sub.3 O.sub.8 oxide, usable as a source of nuclear fuel. These processes are applicable to the recovery of uranium from minerals such as phosphate rock, which also yields phosphoric acid, and from minerals of various origin containing more or less uranium, which is present most frequently in the form of oxides. The prior art processes generally comprise treatment of the mineral with the aid of a strong and concentrated acid, such as sulfuric, phosphoric, hydrochloric or nitric acid, to provide an aqueous solution containing uranyl ions in a highly dilute state, together with other contaminating ions, from which the uranium is then recovered. A typical example of the treatment of such a solution from a raw, wet process phosphoric acid, obtained by the attack of sulfuric acid on phosphate rocks, is described by F. J. Hurst & D. J. Crouse in Ind. Eng. Chem. Process Des. Develop. Vol. 11, No. 1, 1972, pp. 122-128. See also Hurst and Crouse U.S. Pat. No. 3,711,591 and Wiewiorowski et al U.S. Pat. No. 3,737,513 for descriptions of various prior art attempts.
According to the Hurst and Crouse process, the solution in which uranium is found or in which it is transformed to the U.sup.+6 state, is exposed to a first uranium extraction cycle employing an organic solvent consisting of a mixture of synergistic extractants, namely di(2-ethylhexyl) phosphoric acid (designated HDEHP or D2EHPA) and trioctylphosphine oxide (designated TOPO), diluted in a kerosene-type hydrocarbon. The uranium is extracted from the aqueous solution into the organic solvent in the form of a uranyl complex formed between the uranium (VI) UO.sub.2.sup.+2 ions and the synergistic mixture of the extractants. The uranium is subsequently recovered from the organic phase into which it has been extracted by means of contact with an aqueous solution of phosphoric acid containing sufficient iron (II) ions to reduce the uranium (VI) to uranium (IV). Because the quadrivalent state is less extractable, the uranium is transferred to the aqueous phase. This aqueous phase is then reoxidized to return the uranium to the uranium (VI) state of oxidation and is then exposed to a second extraction cycle using an organic phase containing a synergistic mixture of the HDEHP-TOPO extractants to obtain finally, after the re-extraction of uranium with an ammonium carbonate solution, a sufficiently pure mixed uranium and ammonium carbonate.
The prior art process described above has a number of disadvantages at the industrial level. Specifically, the reducing re-extraction in the first cycle requires the addition of iron (II) ions in large amounts, the iron (II) ions being obtained by the action of phosphoric acid on iron, which is a slow and difficult reaction, or by means of the introduction of an iron (II) salt, which involves the introduction of an additional anion. In any case, this operating method has the disadvantage of introducing an appreciable amount of iron into the phosphoric acid, which seriously interferes with the second uranium purification cycle downstream. Furthermore, since the second extraction cycle is to be effected on the oxidized aqueous solution, treatment with an oxidizing agent is necessary. Again, there are attendant difficulties. Thus, if the oxidation is performed with air or with air enriched with oxygen, the operation is slow and requires additional equipment. If oxidation is effected by means of a chemical oxidant, it involves the introduction of harmful foreign ions; for example, the introduction of chlorate ions results, after reduction, in the formation of chloride ions, which are powerful corroding agents. The use of hydrogen peroxide (oxygenated water), another possible oxidant, is very expensive.
Furthermore, once the aqueous phase subjected to reductive re-extraction has been reoxidized and depleted of uranium in the second cycle, it is eliminated from the process. Therefore, practically all of the phosphoric acid introduced, the iron and the chemical oxidizing agent are lost in the process. As an example, taking as the basis for calculation a uranium extraction unit corresponding to the treatment of 300,000 tons per year of P.sub.2 O.sub.5 according to the process described hereinabove, the following estimates ought to be expected:
(a) If ferrous sulfate heptahydrate is chosen as the reducing agent for reasons of low cost and its ready dissolution in phosphoric acid, with the quantity of iron (II) amounting to 28 g per liter of phosphoric acid in the reductive extraction, the daily consumption is of the order of 3.7 tons of ferrous sulfate.7H.sub.2 O. This poses a problem of storage and supply, rendered even more difficult by the fact that the product is hygroscopic and thus difficult to handle. (The calculation takes into consideration the normal partial reoxidation of iron (II) by air in the mixer-decanters.)
(b) If iron metal solubilized by phosphoric acid is used, the consumption will be of the order of 300 kg per day of metal and bulky equipment for dissolution of the metal will be required.
(c) If hydrogen peroxide is chosen as the means for the reoxidation of the phosphoric acid solution containing the uranium (IV) and the iron (II)/iron (III) couple issuing from the first cycle, prior to its entry into the second extraction cycle, the daily consumption of 70% H.sub.2 O.sub.2 is of the order of 150 kg.
In addition, once the preceding aqueous phosphoric acid solution is reoxidized, it enters the second cycle, where the solvent effects the total extraction of uranium and a small portion of the iron. The aqueous solution depleted in uranium is then recycled to the phosphoric acid concentration units or to the top of the primary extraction column and the reducing agent is definitely lost for the subsequent reducing extraction of uranium.
Consequently, a real need exists for a commercially feasible process for the recovery of uranium from impure phosphoric acid which will overcome the problems associated with the prior art procedures.